The present invention relates to a method for continuous quality control of an applied element, such as a staple, with respect to the geometrical, structural and functional parameters of the element. The invention furthermore relates to a measuring device for realizing the method.
As is known, printed products are frequently stitched together with wire staples, wherein a wire-stitching machine is used for this purpose. Wire-stitching machines comprise respectively one stitching head and one bending device for carrying out the stitching operation. This stitching operation involves feeding in a wire, cutting off a section of wire, forming the staple, punching the staple legs through of the product to be stitched and bending over of the two staple legs.
Methods and devices are furthermore known from the prior art which make it possible to check for the presence of staples in a product, so as to remove products with a missing staple. A check of this type can be realized with a metal detector, for example, which is arranged in the transport direction downstream of the wire-stitching machine. Each staple passing through triggers a pulse, wherein a missing pulse is an indication of a missing staple in a product.
A device for detecting a missing staple with the aid of a sensor arranged on a wire-stitching machine is furthermore known from European patent document EP 0 205 144, wherein the sensor is embodied as a proximity switch or as an optical sensor.
However, the aforementioned methods and devices can only be used to check whether a stitching took place, but not the quality of a stitching operation. The method consequently also indicates defective staples as being present or inserted, for example staples having a leg that is projecting outward. Defective stapling operations of this type are undesirable because they can result in injuries to the user and/or the reader of the printed product. Numerous measures have therefore already been proposed according to the prior art for detecting such defective stapling operations and for removing the respective printed products.
European patent document EP 1 029 443 A discloses a device for monitoring the stitching of products, wherein a measuring means is arranged on the wire-stitching machine that is designed to detect changes in the state of a bending device or in a stitching head of the wire-stitching machine which punches the staples through the product. The aforementioned measuring means is embodied, for example, as strain gauges used to detect the force generated during the forming of the staple at the bending device. If this force deviates from a predetermined value, it is assumed that the stitching is faulty and the product is correspondingly removed via the machine control. As a result, the stitching operation in the wire-stitching machine is checked in this case which does not, however, allow drawing conclusions relating to the effective processing of the staple.
A method is furthermore known from European patent document EP 1 591 272 A1 for which the measuring apparatus respectively checks the density course of a staple which is passing through. In the case of defective staples, for example if a leg projects outward or is missing, the density course deviates considerably from that of a faultlessly inserted staple. A faultlessly inserted staple comprises both legs and these are bent as specified, so that generally there is no danger of injury. Other defects in a stitching operation can also be detected with this method, for example even if the staple is a so-called eyelet staple and if the eyelet of the staple is defective, meaning it is bent. This quality-assurance control substantially depends on the degree of resolution of the sensor that is used, especially during the operation.
A method for controlling wire-stitching operations is also known from European document DE 38 17 024 A1. The goal of this method is to increase the quality of the items to be processed and to lower the amount of waste material. A signal image of a wire-stitching operation of a desired product is generated with the aid of a plurality of measuring points, wherein the image is subsequently stored. For quality control, a point-by-point comparison is made with signal values for the wire-stitching of actual products, wherein a faulty wire-stitching is signaled for difference values outside of specified tolerance limits between the desired condition and the actual condition. It is noteworthy for this document that the means for realizing the described pushing direction are summarily mentioned. Thus, with respect to the prior art it is only mentioned therein that an inductive sensor is used for the metal items. Later on in the document, only a sensor is mentioned.
The following facts can therefore be deduced from the prior art:
A coil used for determining the geometric condition of the staple has proven to be too large, so that an approach to the object for a better resolution remains difficult.
Taking the path of using an inductive coil also did not bring the desired success, even though some improvement could be achieved because of the relative miniaturization of the coil. However, the desired resolution could still not be achieved.
The degree of the resolution is thus for the most part predetermined by the physical limits of the sensor that is used.